Merged in jussi_kivilinna/nuttx/stm32l4_serial_pm (pull request #534)

STM32L4 serial PM interface improvements

* stm32l4_serial: pm: check rx/tx buffers for pending data in pmprepare

* stm32l4: remove adhoc PM interfaces and move serial suspend functionality behind CONFIG_PM

Approved-by: Gregory Nutt <gnutt@nuttx.org>